1. Field of the Invention
The present invention relates generally to a projector, and more particularly, to a light source driving apparatus of a projector for sequentially driving a plurality of light sources.
2. Description of the Related Art
As light sources used in a projector, a plurality of light emitting diodes (LEDs) (e.g., red, green, and blue LEDs) are widely used because they can increase optical efficiency compared to light sources employing a color filter scheme. The multiple LED light sources are driven by a color sequential driving scheme. Namely, driving signals of the LED light sources are time-divided and are sequentially switched. For example, red (R), green (G), and blue (B) LEDs are sequentially driven in units of R, G, R, G, and B subframes to constitute one frame, as shown in FIG. 1.
A light source driving circuit used in the color sequential driving scheme typically includes a DC-DC converter (e.g., a buck-boost converter) based 3-MOSFET switching circuit. The light source driving circuit connects a plurality of light sources (e.g., R, G, and B LEDs) having different driving voltages to an output of the DC-DC converter and controls voltages applied to the plurality of light sources using a feedback circuit.
Since the driving circuit controls voltages applied to different light sources using a feedback circuit, if a driving voltage of a light source to be driven next is relatively lower than a driving voltage of a currently driven light source, an overvoltage may be instantaneously applied to the next light source when driving the next light source is started, and thus a peak current may occur.
FIGS. 2A to 2D are timing charts showing voltage waveforms and current waveforms of a general light source driving circuit. Referring to FIGS. 2A to 2D, in typical high brightness LEDs, the driving voltages of red, green, and blue LEDs are about 2.3V, 3.8V, and 3.6V, respectively and a driving current flowing into each LED is about 350 mA. If the red LED is driven after the blue LED is driven, a DC-DC converter may be controlled to output 2.3V from 3.6V during a transition time as shown in FIG. 2A. A difference between the blue LED and the red LED in a driving voltage is about 1.3V. A control signal (refer to FIG. 2B) for driving the red LED is generated during the transition time for driving the red LED, and simultaneously a voltage applied to a feedback circuit of a DC-DC converter is instantaneously increased as shown in FIG. 2C. As a result, a peak current occurs in the red LED as indicated in FIG. 2D.
The peak current may seriously damage a light source and may consume a large amount of driving power. If a driving circuit of a projector is designed to use the same battery as other functional modules (e.g., mobile communication modules), the peak current leads to a voltage drop of the battery, and thus may result in a system error.
FIG. 3 is a block diagram showing a circuit configuration of a conventional LED driver according to the prior art, and FIG. 4 is a block diagram showing an internal configuration of a switch controller illustrated in FIG. 3. To solve the above-described problems, the conventional LED driver (as described in US Patent Publication No. US20060192728 and assigned to the assignee of the present application) bypasses a driving current of a light source during a transition time using a bypass element (refer to reference numeral 16 in FIG. 3). However, the LED driver requires an additional bypass switch driver (refer to reference number 186 in FIG. 4) to control the driving of the bypass element, thereby complicating a driving circuit. In addition, since current continues to flow through the bypass element during the transmission time, power consumption is increased.